Particles useful as toner and/or carrier are prepared by numerous methods, probably the most primitive of which is grinding bulk polymer materials to a suitable particle size and/or particle size distribution. The disadvantages of this process are widely known. Initially, the particles are irregularly shaped after grinding and may not be suitable for their intended purpose. Secondly, some polymeric materials are not amenable to grinding due to their physical properties. Thirdly, the particle size distribution produced by grinding is broad.
In typical copying processes, toner and/or carrier particles are subject to electrostatic and other forces that affect the particles differently depending on their size. Therefore, in order to obtain high quality copies, it is preferred that toner particles have a controlled morphology and narrow size distribution. Past methods of attaining particles of the desired size and size distribution include sizing the toner particles by sieving or air classification. However, these methods result in significant product loss when used to obtain narrow particle size distributions. Recently, to avoid this expensive and time consuming process, new methods have been developed to produce particles having a narrow size distribution and controlled morphology.
One such method, referred to as "evaporation limited coalescence" is described in U.S. Pat. No. 4,833,060 to Nair, et al. ("Nair"). The limited coalescence technique described in Nair involves first forming a solution of a polymer in a solvent that is immiscible with water (along with any required addenda), and then suspending the polymer-solvent solution in water containing a promoter and a colliodal suspension stabilizer. The resulting suspension is subjected to high shear action to reduce the size of the polymer-solvent droplets. The shearing action is removed and the polymer-solvent droplets, with particles of stabilizer at their surface, coalesce to the extent allowed by the stabilizer to form coalesced polymer-solvent droplets. After the drop size has been determined by the limited coalescence, the solvent must be removed from the coalesced drops to form the solidified polymer particles which are then isolated from the suspension by filtration or other suitable means.
The Nair technique is referred herein to as a "conventional" limited coalescence process whereby the particles are formed from a water immiscible solution or dipsersion. In an "inverse" limited coalescence process, the particles are formed from a water or water-miscible solution or dispersion suspended in an immiscible hydrophobic liquid containing a colloidal suspension stabilizer. This inverse method is useful for producing particles having a narrow size distribution and controlled morphology from hydrophilic materials such as precursor to ferrites, ceramic powders, or water soluble polymers. Both techniques are useful to obtain powders with desirable handling properties (e.g., high powder flow, low dusting, etc.). Inverse limited coalescence is described in U.S. patent application Ser. No. 07/888,063 to Lofftus, entitled "INVERSE LIMITED COALESCENCE PROCESSES", filed May 26, 1992.
The Nair limited coalescence process has disadvantages, despite its ability to produce polymeric particles with controlled morphology and narrow size distribution. One key disadvantage is that the solvent is typically removed by evaporation. Evaporation of the solvent or dispersant, is a slow, time consuming process. In addition, many potential limited coalescence systems are not sufficiently robust to withstand the stirring and heating that is required to complete the evaporation process and solidify the coalesced droplets. The agitation and heat destablize the coalesced droplets before they are solidified, causing breakdown of the stabilized interface and agglomeration of the particles. Therefore, there continues to be a need for improved limited coalescence processes.